Position encoders are well known, and, in general, there are three types, absolute, incremental, and quasi-absolute. All of these provide a signal of some form which reflects the position of an object to which they are connected. The principal differences between them is that with quasi-absolute and incremental encoders the object must be moved in order to generate the position signal and the signal must be stored, but with an absolute encoder no motion and signal storage is required, because the signal may be repeatedly generated.
One type of quasi-absolute position encoder is explained in U.S. Pat. No. 4,041,483 to Groff titled ABSOLUTE INCREMENTAL HYBRID SHAFT POSITION ENCODER. Another encoder of this general variety is explained in copending U.S. Patent Application Ser. No. 927,242 to Marvin Masel, et al., titled HIGH RESOLUTION AND WIDE RANGE SHAFT POSITION TRANSDUCER SYSTEMS, which is assigned to the same assignee as this application. Both encoders use first and second gears to rotate associated coded discs. One gear is coupled to a shaft which is connected to the object, and the shaft rotates as the object moves. The other gear is driven by the first gear. As the object moves, the first gear and its disc rotate and the second gear and its disc also rotate. Sensors read the disc codes as the discs rotate, and the sensors produce signals which reflect the angular position of each disc. The number of teeth on the first and second gears are different. Thus, the discs rotate at different rates and the relationship between the angular position of each disc identifies a particular location of the object. By comparing the signals produced during one revolution of each of the discs the location is determined and stored. If power is lost, however, that stored location may be lost. Then, the object must be moved to rotate both discs at least one revolution to update the encoder, to determine the location.
One application for these encoders is in elevator systems to provide signals that reflect elevator car location at the building and that are stored in and used by the system controller. But, with these encoders the location of the car would be unknown following a power failure or a disruption which causes the stored location to be lost, and then, when the system is blind, the car would have to be moved some distance up or down to update the encoder, in order to determine the location of the car. Although this typically does not require much car movement, moving the car when its location is not precisely known is awkward and should be avoided, if at all possible.